From the telegraph.co.uk article:
An amateur British military historian has unearthed a vast underground Nazi gun battery complex thought to have caused carnage during the D-Day landings. Gary Sterne, 43, discovered the huge “Maisy Battery” after he found a crinkled map which fell out of an old pair of US serviceman’s trousers at a military memorabilia fair in Stockport.

It turned out to be an invasion map for Omaha Beach, which included an area marked “area of high resistance”. Mr Sterne, a publisher and collector, believed that this could show the “lost” Nazi gun emplacements, which became buried by nature after the war and could not be located.

Experts were divided about the battery’s location and most believed that the area where Mr Sterne was looking was nothing but fields. But after travelling to Normandy to search for himself he stumbled across an entrance to the complex in undergrowth.

He said: “It sparked my curiosity, because that area was previously thought to be just fields.”

He discovered an extensive installation “the size of four football pitches”, including bunkers, offices, a sizeable field hospital – minus its roof – and housing for 155mm cannon. Trenches surrounding the buildings stretched for a mile and a half.

Within hours of the landings on Jun 6, 1944, at least 2,000 Allied troops are thought to have died. In the days that followed, until the Maisy Battery was captured on Jun 9, hundreds more lost their lives. Mr Sterne said he thought shelling from the battery contributed heavily to Allied losses. It was finally captured following aerial bombardment with 2,000lb bombs.

The Omaha landings and the loss of life that resulted was dramatised in Steven Spielberg’s 1998 blockbuster Saving Private Ryan.